Constructing Soft Perovskite-Substrate Interfaces for Dynamic Modulation of Perovskite Film in Inverted Solar Cells with Over 6200 Hours Photostability

In Situ Cation Exchange Enables Air-Processed Inverted Perovskite Solar Cells with over 25% Efficiency and Enhanced Stability

The scalable fabrication of inverted perovskite solar cells (IPSCs) in humid air with an antisolvent-free process is essential for future industrial applications. However, high humidity poses significant challenges for achieving high-quality perovskite films, making it difficult to attain efficient IPSCs under ambient conditions. Here, we present an in situ cation exchange strategy to create a compact and uniform PbI₂ shell on the perovskite surface by using ZnI₂ in acetonitrile (ACN), where Zn2+ replaces Pb2+. This transformation is attributed to the surface defects of the perovskite, which undergo a cation exchange reaction in humid air, forming a compact PbI₂ shell. The n-type PbI₂ shell effectively encapsulates the perovskite films, minimizing air exposure while optimizing energy level alignment, thereby enhancing electron transport and extraction. As a result, we demonstrate IPSCs with efficiencies of 25.2% under ambient conditions (25°-30 °C, 60% ± 10% relative humidity [RH]), on par with state-of-the-art devices fabricated in inert atmospheres. The devices demonstrated remarkable stability, enduring aging tests under the International Summit on Organic Photovoltaic Stability (ISOS) protocols ISOS-D-1I and ISOS-D-2I for over 4770 and 2000 h, respectively.

PTAA-Based Perovskite Photovoltaics Catching up: Ionic Liquid Engineering-Assisted Crystallization Through Sequential Deposition

PTAA as a widely studied polymeric hole transporting material, has garnered significant attention due to its outstanding thermal and chemical stability. However, the performance of PTAA-based p-i-n devices is shown to lag behind counterpart utilizing oxides or SAMs. In this study, the ionic liquid, 1-ethyl-3-methylimidazolium formate (EMIMCOOH), is innovatively introduced into the lead iodide (PbI2) precursor solution, resulting in a more pronounced mesoporous PbI2 film with expended pore-size and denser pores. This enhancement is attributed to the coordination bond between the ─C═O group in EMIMCOOH and Pb2+. This intensified mesoporous morphology not only facilities the reaction between PbI2 and the organic layer, but also promotes the PbI2 conversion into perovskite material. Importantly, the incorporation of EMIMCOOH slows down the perovskite conversion process, increasing perovskite domain size and suppressed Pb0 trap density, resulting in a uniform perovskite layer with enhanced charge transport properties, as evidenced by the conducting atomic force microscope (c-AFM) results. As a result, the incorporation of EMIMCOOH yields a power conversion efficiency (PCE) of 24.10% and a high fill factor exceeding 85%. Notably, the PCE of the EMIMCOOH-modified device can still maintain 86% of the initial value after 1500 h at 25 °C in an N2 atmosphere.

Iodine Stabilization in Perovskite Lattice for Internal Stress Relief

Atomic iodine ionization in perovskite crystals leads to defect formation, lattice distortion, and the occurrence of localized micro-strain. These atomic-level chemical and mechanical effects significantly alter the electronic band landscape, profoundly affecting device performance. While iodine stabilization effects have traditionally been focused on stability, their impact on electrical properties, particularly the coupling effect with internal stress and lattice strain, remains underexplored. In this study, an iodine stabilization protocol using a parallel-π-stacked small molecule, [2,2]-paracyclophane (PCP) is implemented, which plays a beneficial role in relieving internal stress within the perovskite lattice, thereby improving the film's electrical properties. By leveraging this iodine stabilization strategy, internal stress in the perovskite film, resulting in a strain-free perovskite film and a corresponding device with an improved efficiency of 25.26% from 23.93% is successfully alleviated. The maximum power point tracking test of the perovskite device keeps 85% of its initial efficiency when illuminated under 1 sun for 1000 h, while the control device only maintains 57% of the initial efficiency under the same conditions. The good stability originates from the stable iodide ions in the perovskite lattice due to preventing iodide ions oxidation and perovskite degradation.

Polymeric Charge-Transporting Materials for Inverted Perovskite Solar Cells

Inverted perovskite solar cells (PSCs) hold exceptional promise as next-generation photovoltaic technology, where both perovskite absorbers and charge-transporting materials (CTMs) play critical roles in cell performance. In recent years, polymeric CTMs have played an important role in developing efficient, stable, and large-area inverted PSCs due to their unique properties of high conductivity, tunable structures, and mechanical flexibility. This review provides a comprehensive overview of polymeric CTMs used in inverted PSCs, encompassing polymeric hole transport materials (HTMs) and electron transport materials (ETMs). the relationship between their molecular structures, modification strategies are systematically summarized and analyzed for adjusting energy levels, and improving charge extraction, enabling a deep understanding of these widely used materials. The review also explores effective strategies for designing even more efficient polymeric CTMs. Finally, an outlook is proposed on the exciting research of novel polymeric CTMs, paving the way for their commercialized applications in PSCs.

Synergistic Modulation of Sn2+ Oxidation and Perovskite Crystallization Induced by 4-Hydroxypyridine for Stable Lead-Free Solar Cells

Tin perovskites present promising alternatives to lead perovskites, offering comparable optoelectronic properties alongside environmentally friendly characteristics. However, the rapid crystallization and easy oxidation of Sn2+ lead to poor film quality, further constraining the device performance. Here, 4-hydroxypyridine (4-HP) is introduced into the tin perovskite precursor for fabrication of high-quality tin perovskite films. 4-HP could modulate the colloidal size of prenucleation perovskite clusters in the precursor, thus inducing fast nucleation and retarding the crystal growth rate of tin perovskite through the formation of chemical interaction between nitrogen of pyridine and Sn2+ ions. Furthermore, the hydroxyl group on the pyridine ring contributes to suppressing the oxidation of Sn2+. As a result, the power conversion efficiency (PCE) of the devices based on 4-HP increases up to 11.3%. The stability of the unencapsulated devices shows significant improvement, retaining 100% of their initial PCEs after 2000 h of storage in N2 with 50-100 ppm of O2. This research presents a novel approach to the synchronized regulation of tin perovskite crystallization and the suppression of Sn2+ oxidation.

Spontaneous Compositional-Interfacial Co-Modification Engineering via Ion Exchange Reaction Between Perovskite and Electron-Transporting Layer for Exceptionally Long-Term Stability of Photovoltaics

The long-term stability of perovskite solar cells (PSCs) is still challenging for commercialization and mainly linked to the life span of perovskite films. Herein, a spontaneous compositional-interfacial co-modification strategy is developed based on the ion exchange reaction by introducing ammonium hexafluorophosphate (NH4PF6) into antisolvent to form gradient structures through a simple one-step solvent engineering. With the assistance of the ion exchange reaction, NH4PF6 forms a multifunctional structure to protect perovskite films from both internal and external factors for the exceptionally long-term stability of photovoltaics. The reason for this is linked to the high hydrophobicity of NH4PF6 for preventing H2O invasion, suppressing ion migration by forming hydrogen bonding, and reducing perovskite defects. The resulting unencapsulated devices show exceptionally long-term stability under standardized the International Summit on Organic Photovoltaic Stability (ISOS) protocols, with over 94%, 81%, and 83% retained power conversion efficiencies after aging tests under N2 (ISOS-D-1I), ambient air (ISOS-D-1), and 85 °C (ISOS-D-2I) for 14016, 2500, and 1248 h, respectively. These performances compare well with the state-of-the-art stability of inverted PSCs. Further investigations are conducted to study the evolution of macroscopic morphology and microscopic crystal structure in aged perovskite films, aiming to provide evidence supporting the aforementioned improvements in stability.

Application of Strain Engineering in Solar Cells

Solar cells represent a promising innovation in energy storage, offering not only exceptional cleanliness and low cost but also a high degree of flexibility, rendering them widely applicable. In recent years, scientists have dedicated substantial efforts to enhancing the performance of solar cells, aiming to drive sustainable development and promote clean energy applications. One approach that has garnered significant attention is strain engineering, which involves the adjustment of material microstructure and organization through mechanical tensile or compressive strain, ultimately serving to enhance the mechanical properties and performance stability of materials. This paper aims to provide a comprehensive review of the latest advancements in the application of strain engineering in solar cells, focused on the current hot research area-perovskite solar cells. Specifically, it delves into the origins and characterization of strain in solar cells, the impact of strain on solar cell performance, and the methods for regulating stable strain. Furthermore, it outlines strategies for enhancing the power conversion efficiency (PCE) and stability of solar cells through strain engineering. Finally, the paper conducts an analysis of the challenges encountered in the development process and presents a forward-looking perspective on further enhancing the performance of solar cells through strain engineering.

Strain Engineering and Halogen Compensation of Buried Interface in Polycrystalline Halide Perovskites

Inverted perovskite solar cells based on weakly polarized hole-transporting layers suffer from the problem of polarity mismatch with the perovskite precursor solution, resulting in a nonideal wetting surface. In addition to the bottom-up growth of the polycrystalline halide perovskite, this will inevitably worse the effects of residual strain and heterogeneity at the buried interface on the interfacial carrier transport and localized compositional deficiency. Here, we propose a multifunctional hybrid pre-embedding strategy to improve substrate wettability and address unfavorable strain and heterogeneities. By exposing the buried interface, it was found that the residual strain of the perovskite films was markedly reduced because of the presence of organic polyelectrolyte and imidazolium salt, which not only realized the halogen compensation and the coordination of Pb2+ but also the buried interface morphology and defect recombination that were well regulated. Benefitting from the above advantages, the power conversion efficiency of the targeted inverted devices with a bandgap of 1.62 eV was 21.93% and outstanding intrinsic stability. In addition, this coembedding strategy can be extended to devices with a bandgap of 1.55 eV, and the champion device achieved a power conversion efficiency of 23.74%. In addition, the optimized perovskite solar cells retained 91% of their initial efficiency (960 h) when exposed to an ambient relative humidity of 20%, with a T80 of 680 h under heating aging at 65 °C, exhibiting elevated durability.

Enhanced Resonance for Facilitated Modulation of Large-Area Perovskite Films with Stable Photovoltaics

Upscaling efficient and stable perovskite films is a challenging task in the industrialization of perovskite solar cells partly due to the lack of high-performance hole transport materials (HTMs), which can simultaneously promote hole transport and regulate the quality of perovskite films especially in inverted solar cells. Here, a novel HTM based on N-C = O resonance structure is designed for facilitating the modulation of the crystallization and bottom-surface defects of perovskite films. Benefiting from the resonance interconversion (N-C = O and N+ = C-O- ) in donor-resonance-donor (D-r-D) architecture and interactions with uncoordinated Pb2+ in perovskite, the resulting D-r-D HTM with two donor units exhibits not only excellent hole extraction and transport capacities, but also efficient crystallization modulation of perovskite for high-quality photovoltaic films in large area. The D-r-D HTM-based large-area (1.02 cm2 ) devices exhibit high power conversion efficiencies (PCEs) up to 21.0%. Moreover, the large-area devices have excellent photo-thermal stability, showing only a 2.6% reduction in PCE under continuous AM 1.5G light illumination at elevated temperature (≈65 °C) for over 1320 h without encapsulation.

Bridging the Buried Interface with Piperazine Dihydriodide Layer for High Performance Inverted Solar Cells

Given that it is closely related to perovskite crystallization and interfacial trap densities, buried interfacial engineering is crucial for creating effective and stable perovskite solar cells. Compared with the in-depth studies on the defect at the top perovskite interface, exploring the defect of the buried side of perovskite film is relatively complicated and scanty owing to the non-exposed feature. Herein, the degradation process is probed from the buried side of perovskite films with continuous illumination and its effects on morphology and photoelectronic characteristics with a facile lift-off method. Additionally, a buffer layer of Piperazine Dihydriodide (PDI₂ ) is inserted into the imbedded bottom interface. The PDI₂ buffer layer is able to lubricate the mismatched thermal expansion between perovskite and substrate, resulting in the release of lattice strain and thus a void-free buried interface. With the PDI₂ buffer layer, the degradation originates from the growing voids and increasing non-radiative recombination at the imbedded bottom interfaces are suppressed effectively, leading to prolonged operation lifetime of the perovskite solar cells. As a result, the power conversion efficiency of an optimized p-i-n inverted photovoltaic device reaches 23.47% (with certified 23.42%) and the unencapsulated devices maintain 90.27% of initial efficiency after 800 h continuous light soaking.

Multifunctional Resonance Bridge-Mediated Dynamic Modulation of Perovskite Films For Enhanced Intrinsic Stability of Photovoltaics

The improving intrinsic stability, determining the life span of devices, is a challenging task in the industrialization of inverted perovskite solar cells. The most important prerequisite for boosting intrinsic stability is high-quality perovskite films deposition. Here, a molecule, N-(2-pyridyl)pivalamide (NPP) is utilized, as a multifunctional resonance bridge between poly(triarylamine) (PTAA) and perovskite film to regulate the perovskite film quality and promote hole extraction for enhancing the device intrinsic stability. The pyridine groups in NPP couple with the phenyl groups in PTAA through π-π stacking to improve hole extraction capacities and minimize interfacial charge recombination, and the resonance linkages (NCO) in NPP dynamically modulate the perovskite buried defects through strong PbO bonds based on the fast self-adaptive tautomerization between resonance forms (NCO and N⁺ CO^- ). Because of the combined effect of the reduction defect density and improved energy level in the perovskite buried interfaces as well as the optimized crystal orientation in perovskite film enabled by the NPP substrate, the devices based on NPP-grown perovskite films show an efficiency approaching 20% with negligible hysteresis. More impressively, the unencapsulated device displays start-of-the-art intrinsic photostability, operating under continuous 1-sun illumination for 2373 h at 65 °C without loss of PCE.